Belonging to the simplest prior art DC/DC converters are the so-called inductor-type converters serving to achieve a practically lossless potential conversion of a DC voltage into a desired output DC voltage with the aid of an inductance. One of the basic types of the inductor-type converters is the so-called step-up converter with which an input DC voltage can be converted into a boosted output DC voltage whose value exceeds that of the input DC voltage.
One such step-up converter is described e.g. in the German book on switching power supplies “Schaltnetzteile” by Udo Leonhard Thiel, 2nd Edition, published 1998 by Franzis-Verlag, on pages 34 et seq. In this described step-up converter, an input voltage is applied to the one terminal of an inductance, the other terminal of which can be connected via a controllable switch alternatingly to ground or a capacitor provided at the output of the circuit and the consumer respectively. In this arrangement, it is the duty cycle of the switch that determines the value of the output voltage. Inserted between the other terminal of the inductance and the capacitor is a diode preventing the backflow of current from the output of the regulator to the input. To devise the circuit, the current-saving diode is replaced as a rule by two controllable switches for the first of which a MOS-FET is used and for the second switch a PMOS-FET.
One such circuit in which two MOS-FETs are used as the controllable switches is illustrated in FIG. 1 wherein the NMOS-FET is identified by N1 and the PMOS-FET by P1. The inductance L is connected by its one terminal to the input voltage Vin of the DC/DC converter and by its other terminal to the drain of the NMOS-FET whose source is connected to ground potential Vss. The other terminal of the inductance L is additionally connected to the drain of the PMOSFET P1 whose source is connected to the output of the circuit. The back gate of the PMOS-FET is connected to the output voltage Vout. Provided further is a regulation circuit 1 which monitors the output voltage Vout and alternatingly connects the switches N1 and P1, the duty cycle being set so that the wanted value of the output voltage is attained. In this arrangement, the output voltage Vout and ground potential is alternatingly applied to the terminals of the two MOS-FETs.
Provided further at the output of the circuit is a storage capacitor C.
One problem associated with the simple circuit as shown in FIG. 1 is that it can only be put to use for step-up but not for step-down voltage regulation. When in the circuit as shown in FIG. 1, the value of the input voltage Vin exceeds the wanted value of the output voltage by a transistor threshold voltage, a current flows via the back gate of the PMOS-FET P1 which charges the output capacitor C to a value higher than the wanted value of the output voltage thus making stepdown regulation impossible. The circuit as shown in FIG. 1 is thus not suitable e.g. for battery-powered devices in which the battery voltage during the lifetime of the battery initially exceeds the wanted operating voltage of a device powered with a voltage regulator so that it needs to be bucked (stepped down), and later after a certain discharge of the battery, drops below this wanted voltage so that it needs to be boosted (stepped up). For such applications, only relatively complicated converters such as buck+boost or SEPIC converters come into question necessitating a costly array of external components.